px4_log: Fixed compiler warning when using PX4_LOG

If __px4_log_level_current is unsigned then the runtime filter
comparison warns because an unsigned value can't be less than zero.

Changed typed to signed so compiler will not issue a warning.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/platforms/common/px4_log.c

 #include <px4_log.h>
 
-__EXPORT unsigned int __px4_log_level_current = PX4_LOG_LEVEL_AT_RUN_TIME;
+__EXPORT int __px4_log_level_current = PX4_LOG_LEVEL_AT_RUN_TIME;
 
 __EXPORT const char *__px4_log_level_str[_PX4_LOG_LEVEL_DEBUG+1] = { "INFO", "PANIC", "ERROR", "WARN", "DEBUG" };

src/platforms/px4_log.h

@@ -65,7 +65,7 @@ __EXPORT extern uint64_t hrt_absolute_time(void);
 #define _PX4_LOG_LEVEL_DEBUG		4
 
 __EXPORT extern const char *__px4_log_level_str[5];
-__EXPORT extern unsigned int __px4_log_level_current;
+__EXPORT extern int __px4_log_level_current;
 
 // __px4_log_level_current will be initialized to PX4_LOG_LEVEL_AT_RUN_TIME
 #define PX4_LOG_LEVEL_AT_RUN_TIME	_PX4_LOG_LEVEL_WARN